To drive a rotatable load, it is well known to use a motor and a drive system. The drive system transmits the power supplied by the motor to the load, and transforms the motor torque and speed to the torque and speed required by the load; usually the load requires a higher torque than the one provided by the motor, but at a lower speed. In a number of steps, called transmission steps, the motor power is transmitted to the load, while each transmission step provides a lower speed and at the same time a higher torque. An example of a drive system is the gearbox of a car--or the automatic transmission, in which case the gear is changed automatically.
Common drive systems comprise mechanical components such as gears, belts, chains, or combinations thereof.
The purpose of a drive system is driving the load at a chosen angular speed, keeping this speed substantially constant, in spite of load variations, and minimizing shocks experienced by the load, i.e. sudden decelerations or accelerations. Shocks can be caused by the motor, e.g. a stepper motor creates accelerations and decelerations; they can be caused by load variations; they can also be introduced by the drive system itself.
A drive system consisting of gear transmissions creates shocks at every engagement and disengagement of the teeth.
By making use of a timing belt, the engagement and the disengagement of the teeth of the belt with its belt pulleys causes minute accelerations and decelerations.
The extent of accelerations and decelerations of course depends on what the load can tolerate. By way of example, hereinafter the drive system of the print drum of a thermal printer will be described. In this case, most accelerations and decelerations of the load, i.e. the print drum, are visible in the image produced by the printer. Whether a shock on the print drum is visible or not in the printed image depends on its amplitude and its frequency.
Thermal imaging or thermography is a recording process wherein images are generated by the use of image-wise modulated thermal energy.
In thermography two approaches are known:
1. Direct thermal formation of a visible image pattern by the image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. PA1 2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element into a receptor element.
Common thermal printers comprise a rotatable drum and an elongated thermal head which is spring-biased towards the drum to firmly line-wise contact a heat-sensitive material which is passed between the head and the drum.
The thermal head includes a plurality of heating elements and corresponding drivers and shift registers for these elements. The image-wise heating of a sheet is performed on a line by line basis. The heating resistors are geometrically juxtaposed along each other in a bead-like row running parallel to the axis of the drum. Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element.
In direct thermal image formation, a single heat-sensitive sheet is conveyed between the thermal head and the drum, and the image is directly produced on the sheet. The sheet is not attached to the drum but is advanced between the head and the drum by frictional contact of its rearside with the drum.
Medical diagnostics are an application area of direct thermal printing; here an image is produced on a transparent sheet, a polyethylene terephthalate support in particular.
In thermal dye transfer, the sheet--i.e. the image receiving sheet--is usually attached to the rotatable drum, and a dye donor sheet or web is conveyed by frictional contact with the rotating sheet past the thermal head.
In practising the thermal printing technique described hereinbefore, the image quality may be spoiled by a defect which will be called "banding" hereinafter, and which is characterized by transverse zones (i.e. parallel with the thermal head) on the final print of slightly increased and/or reduced optical density which are particularly visible in the areas of lower optical density, say smaller than 1.0.
A known cause for this type of defect is the drive system for the drum. The drive system can cause minute accelerations and decelerations, leading to corresponding reductions and prolongations of the printing time.
In a known prior art system, the print drum is driven by a sequence of timing belts: the motor drives a first intermediate shaft via a first timing belt, this first intermediate shaft drives a second intermediate shaft via a second timing belt, and the second intermediate shaft drives the print drum via a third timing belt. Since the engagement and the disengagement of the teeth of each timing belt with its belt pulleys causes minute accelerations and decelerations, such a drive system contributes significantly to the banding defect.
We have found that severe banding can occur when a substantially white line in the image is followed by a substantially black line. In this case, the resistors in the thermal head very suddenly have to provide a large quantity of thermal energy. When writing the substantially white line, the heat-sensitive material is nearly not heated, while when writing the substantially black line, a large quantity of heat is supplied by the resistors in the thermal head. As a consequence, the thermal head sinks into the heat-sensitive material, thus causing a large increase of the friction force between thermal head and heat-sensitive material. This increase of friction, which will be called "frictional shock" hereinafter, causes an important deceleration of the print drum.
Banding created by the timing belts is an example of shocks caused by the drive system, whereas the frictional shock is an example of a shock caused by a load variation.
Both types of shocks cause defects that are visible in the printed image.
Besides the problem of shocks, a second problem in drive systems is keeping the angular speed of the load substantially constant, in spite of load variations. A stepper motor drives its load at a constant speed; however, as mentioned hereinbefore, a stepper motor creates shocks itself. A DC-motor operates substantially shock-free, but the speed of a DC-motor changes considerably with load variations. A known solution to this problem is to place a rotational encoder on the same shaft as the load. The encoder converts the angular position of the load into a drive signal; this drive signal is used to drive the DC-motor. If the angular speed of the load decreases, less pulses per second are registered by the encoder, and the drive signal to the motor is adjusted accordingly.
A drawback of this method is the cost of the encoder: to measure the angular position of the load with sufficient accuracy, a high resolution encoder is required, providing a very high number of pulses per revolution of the load.